Panasonic VL-SV75 Series Service Manual page 10

Call Signal from Door station:
When the call button is pressed in Stand-by mode, the short between lines causes the DC voltage of the interphone cable to
go down. The monitor station detects this voltage reduction and then rings. Then LCD of the Monitor station is turned ON.
Fig. 4.2. Call signal from Door station
Communication to Extension monitor:
When the call is detected by Monitor station.
The Main CPU command of Monitor station is modulated to ASK signal and sent to Extension monitor.
This ASK signal is demodulated by Extension monitor.
Extension monitor detects this call and then rings.
Then LCD of Extension monitor is turned ON.
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